142 research outputs found
Stellar Bar Evolution in Cuspy and Flat-Cored Triaxial CDM Halos
We analyze the evolution of stellar bars in galactic disks in mildly triaxial
flat-core and cuspy CDM halos. We use tailored simulations of rigid and live
halos which include the feedback from disk/bar onto the halo in order to test
the work by El-Zant & Shlosman (2002). The latter used the Liapunov exponents
to analyze the fate of bars in analytical asymmetric halos. We find: (1) The
bar growth is similar in all rigid axisymmetric and triaxial halos. (2) Bars in
live models vertically buckle and form a pseudobulge with a boxy/peanut shape.
(3) In live axisymmetric halos, the bar strength varies little during the
secular evolution. The bar pattern speed anticorrelates with the halo core
size. The bar strength is larger for smaller disk-to-halo mass ratios within
disk radii, the bar size correlates with the halo core sizes, and the bar
pattern speeds -- with the halo central mass concentration. Bars embedded in
live triaxial halos have a starkly different fate: they dissolve on ~1.5-5 Gyr
due to the onset of chaos over continuous zones, leaving behind a weak oval
distortion. The onset of chaos is related to the halo triaxiality, the fast
rotating bar and the halo cuspiness. Before the bar dissolves, the region
outside it develops strong spiral structures, especially in the live triaxial
halos. (4) More angular momentum is absorbed by the triaxial halos as compared
to the axisymmetric models and its exchange is mediated by resonances. (5)
Cuspy halos are more susceptible than flat-core halos to having their
prolateness washed out by the bar. We analyze these results in terms of the
stability of trajectories and development of chaos. We set constraints on the
triaxiality of DM halos by comparing our predictions to recent observations of
bars out to z~1.Comment: 17 pages, 14 figures, Astrophysical Journal, in press, Vol. 637.
Updated version (text, references
Star formation in isolated AMIGA galaxies: dynamical influence of bars
Star formation depends strongly both on the local environment of galaxies,
and on the internal dynamics of the interstellar medium. To disentangle the two
effects, we obtained, in the framework of the AMIGA project, Ha and Gunn r
photometric data for more than 200 spiral galaxies lying in very low-density
regions of the local Universe. We characterise the Ha emission, tracing current
star formation, of the 45 largest and less inclined galaxies observed for which
we estimate the torques between the gas and the bulk of the optical matter. We
could subsequently study the Ha morphological aspect of these isolated spiral
galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and
also on the strength of the bars, computing the torques between the gas and
newly formed stars (Ha) and the bulk of the optical matter (Gunn r). We
interpret the various bar/spiral morphologies observed in terms of the secular
evolution experienced by galaxies in isolation. We also classify the different
spatial distributions of star forming regions in barred galaxies. The observed
frequency of particular patterns brings constraints on the lifetime of the
various evolution phases. We propose an evolutive sequence accounting for the
transitions between the different phases we could observe. Isolated galaxies
appear not to be preferentially barred or unbarred. Through numerical
simulations, trying to fit the Ha distributions yields constraints on the star
formation law, which is likely to differ from a genuine Schmidt law. In
particular, it is probable that the relative velocity of the gas in the bar
also needs to be taken into account.Comment: 15 pages, 9 figures (low resolution), 2 tables, accepted by A&
Значенння орієнталізма в музичній творчості Ц.А. Кюі і М.П. Мусоргського
Целью статьи является анализ ориентального творчества композиторов Ц.А. Кюи и М.П. Мусоргского. Исходя из целей статьи, были поставлены следующие задачи:
1. Анализ и оценка ориентального творчества Ц.А. Кюи. 2. Анализ и оценка ориентального творчества М.П. Мусоргского
Variation of Bar Strength with Central Velocity Dispersion in Spiral Galaxies
We investigate the variation of bar strength with central velocity dispersion
in a sample of barred spiral galaxies. The bar strength is characterized by
, the maximal tangential perturbation associated with the bar, normalized
by the mean axisymmetric force. It is derived from the galaxy potentials which
are obtained using near-infrared images of the galaxies. However, is
sensitive to bulge mass. Hence we also estimated bar strengths from the
relative Fourier intensity amplitude () of bars in near-infrared images.
The central velocity dispersions were obtained from integral field spectroscopy
observations of the velocity fields in the centers of these galaxies; it was
normalized by the rotation curve amplitude obtained from HI line width for each
galaxy. We found a correlation between bar strengths (both and )
and the normalized central velocity dispersions in our sample. This suggests
that bars weaken as their central components become kinematically hotter. This
may have important implications for the secular evolution of barred galaxies.Comment: To appear in Ap&S
MYRIAD: A new N-body code for simulations of Star Clusters
We present a new C++ code for collisional N-body simulations of star
clusters. The code uses the Hermite fourth-order scheme with block time steps,
for advancing the particles in time, while the forces and neighboring particles
are computed using the GRAPE-6 board. Special treatment is used for close
encounters, binary and multiple sub-systems that either form dynamically or
exist in the initial configuration. The structure of the code is modular and
allows the appropriate treatment of more physical phenomena, such as stellar
and binary evolution, stellar collisions and evolution of close black-hole
binaries. Moreover, it can be easily modified so that the part of the code that
uses GRAPE-6, could be replaced by another module that uses other
accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate
choice of the free parameters give a good accuracy and speed for simulations of
star clusters up to and beyond core collapse. Simulations of Plummer models
consisting of equal-mass stars reached core collapse at t~17 half-mass
relaxation times, which compares very well with existing results, while the
cumulative relative error in the energy remained below 0.001. Also, comparisons
with published results of other codes for the time of core collapse for
different initial conditions, show excellent agreement. Simulations of King
models with an initial mass-function, similar to those found in the literature,
reached core collapse at t~0.17, which is slightly smaller than the expected
result from previous works. Finally, the code accuracy becomes comparable and
even better than the accuracy of existing codes, when a number of close binary
systems is dynamically created in a simulation. This is due to the high
accuracy of the method that is used for close binary and multiple sub-systems.Comment: 24 pages, 29 figures, accepted for publication to Astronomy &
Astrophysic
The large asymmetric HI envelope of the isolated galaxy NGC 864 (CIG 96)
We present a HI synthesis imaging study of NGC 864 (CIG 96), a spiral galaxy
well isolated from similarly sized companions, yet presenting an intriguing
asymmetry in its integral HI spectrum. The asymmetry in the HI profile is
associated with a strong kinematical perturbation in the gaseous envelope of
the galaxy, where at one side the decay of the rotation curve is faster than
keplerian. We detect a small (M(HI) = 5 x 10^6 Msun) galaxy with a faint
optical counterpart at around 80 kpc projected distance from NGC 864. This
galaxy is probably not massive enough to have caused the perturbations in NGC
864. We discuss alternatives, such as the accretion of a gaseous companion at a
radial velocity lower than the maximum.Comment: 4 pages, 7 figures; Astronomy & Astrophysics, in press; comments by
the referee include
A giant bar induced by a merger event at z=0.4?
(Aims) We present a physical model of the formation of J033239.72-275154.7, a
galaxy observed at z=0.41 and characterized by a big young bar of size 6 kpc.
The study of this system is particularly interesting for understanding the
connection between mergers and bars as well as the properties and fate of this
system as it relates to disk galaxy formation. (Methods) We compare the
morphological and kinematic properties of J033239.72-275154.7, the latter
obtained by the GIRAFFE spectrograph, to those derived from the merger of two
spiral galaxies described by idealized N-body simulations including a star
formation prescription. (Results) We found that the general morphological shape
and most of the dynamical properties of the object can be well reproduced by a
model in which the satellite is initially put in a retrograde orbit and the
mass ratio of the system is 1:3. In such a scenario, a bar forms in the host
galaxy after the first passage of the satellite where an important fraction of
available gas is consumed in an induced burst. In its later evolution, however,
we find that J033239.72-275154.7, whose major progenitor was an Sab galaxy,
will probably become a S0 galaxy. This is mainly due to the violent relaxation
and the angular momentum loss experienced by the host galaxy during the merger
process, which is caused by the adopted orbital parameters. This result
suggests that the building of the Hubble sequence is significantly influenced
by the last major collision. In the present case, the merger leads to a severe
damage of the disk of the progenitor, leading to an evolution towards a more
bulge dominated galaxy.Comment: 6 pages, 5 figures. Accepted for publication in A&
Galaxy Zoo: CANDELS barred discs and bar fractions
The formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z ~ 1, and by z > 1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z ~ 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 (fbar = 10.7+6.3 -3.5 per cent after correcting for incompleteness) does not significantly evolve.We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion yearsPeer reviewedFinal Accepted Versio
- …